Hydraulic fracturing is widely used for stimulating petroleum and gas production and recovery from subterranean formations.
It involves the injection of a suitable fluid down a wellbore to reach a formation; the fluid shall be injected under sufficient pressure to extensively crack the formation and to provide passageways for the oil and gas that are contained in the pores spaces of the formation and help them flowing to the wellbore. Suitable particulate materials (proppants) are often injected into the formation to prevent the closure of the fractures.
Usually, fracturing fluids are gelled with water soluble polymers, especially with natural polymers or chemically modified natural polymers, such as etherified natural polymers, to most effectively widen the fractures and inhibit fluid loss, Water soluble polymers are mainly available in powder form and must be dissolved in the aqueous fluid to perform their viscosifying function.
Dissolution of natural polymer particles in aqueous fluids is typically accompanied by the formation of lumps; upon contact with water, a thin, sticky layer of gel forms on the surface of the particles preventing water from hydrating the inner part of the particles and favoring the formation of lumps.
As a consequence, the whole hydration step of the polymer is undesirably prolonged, especially if the polymer shall be dissolved in large amounts of saline aqueous fluids, which often happens in the preparation of aqueous fracturing fluids.
Among the natural polymers that are used to thicken fracturing fluids, polygalactomannan gums and chemically modified polygalactomannan are widely used, because they form strong gels in combination with crosslinkers that are usually based on titanium, zirconium and boron salts.
Polygalactomannans (also called galactomannans) are polysaccharides obtained from the endosperms of leguminosae seeds consisting of a mannose backbone with galactose side groups (more specifically, a (1-4)-linked beta-D-mannopyranose backbone with branchpoints from their 6-positions linked to alpha-D-galactose, i.e. 1-6-linked alpha-D-galactopyranose).
To provide a gelled fracturing fluid, polygalactomannan gums and chemically modified polygalactomannans shall be previously dissolved in the aqueous component of the fluid and then crosslinked with a crosslinking composition. Unfortunately, also the dissolution of polygalactomannan gums and chemically modified polygalactomannans, such as etherified polygalactomannans, suffer from the disadvantages described above, i.e. the formation of lumps.
Many solutions have been put into practice to avoid lumping in the preparation of fracturing fluids, including apparatus that are specifically designed to hydrate the polymers and to continuously produce viscous treatment gel close to the oil well site, as it is known from US 2006/107998.
Another way to rapidly hydrate the viscosifying polymers is to prepare a concentrated slurry of the polymer in a non-aqueous carrier fluid, usually a hydrocarbon fluid, glycols or glycol derivatives, which facilitates the polymer dispersion and slurry mixing, but may represent a concern for the environment and an additional cost.
It is also well known in the art (by way of example from U.S. Pat. No. 5,165,479) to treat natural gums with small amounts of crosslinking agents, such as glyoxal, borates and the like, to inhibit hydration and minimize the formation of lumps upon contact with water.
U.S. Pat. No. 3,808,195, by way of example, describes a process for rendering polygalactomannans and their water-soluble derivatives dispersible comprising treating the gums and gum derivatives with boron salts.
U.S. Pat. No. 3,297,583 describes a method for the rapid and lump-free dissolution of many macromolecular substances that involves the use of from 0.005% to 5% by weight of aldehydes as crosslinking agents to improve dispersibility.
Although treatment with crosslinking agents is well known to improve dispersibility of polygalactomannans, the net result of the treatment with crosslinking agents disclosed by the prior art are polymers that are dispersible but have a dissolution time which may still be unsuitable for fracturing operations.
Therefore, it would be highly desirable to provide a polygalactomannan which is readily soluble and can be used as viscosifying agent for aqueous based fracturing fluids because of its fast dissolving characteristics and dispersibility.
The polygalactomannan ethers commonly used in the field are purified products, i.e. the reaction by-products have been removed by washing the reaction mass, so that they usually contain more than 95% by weight of active matter.
It has now been found that unpurified polygalactomannan ethers are more easily dispersible and fast dissolving in fracturing fluid when compared to the corresponding purified products. Unexpectedly, their impurities beneficially affect their dissolution rate and their dispersibility without impairing the characteristics required as viscosifying agent.
With the expression “unpurified polygalactomannan ethers” we mean polygalactomannan ethers which have not been washed after the etherification and contain from 4 to 25% by dry weight of the etherification reaction by-products. The amount of the etherification reaction by-product (impurities) is measured as the ash content, for carboxyalkyl or cationic ethers of polygalactomannans, and as the sum of the ash content and the glycol and polyglycol content for hydroxyalkyl or mixed ethers of polygalactomannans. In the present text, with the expression “molar substitution” (MS), we mean the average number of hydroxyalkyl substituents on each anhydroglycosidic unit of the polygalactomannan, which can be measured, for example, by 1H-NMR.
With the expression “degree of substitution” (DS), we mean the average number of substituted hydroxyl groups on each anhydroglycosidic unit of the polygalactomannan, which can be measured, for example, by 1H-NMR.